So there's a problem with my MBP and I sent it to Genius bar a few days ago. They told me I've got some minor liquid damage and agreed to replace some parts of MBP. I just got it back yesterday.

What I notice is everything seems to be normal except for the Windows drivers are missing. I mean they are still there, they still exist in the Control Panel but seems like Windows couldn't detect the hardware. I re-install and everything works normally.

I'm just curious about which part in my MBP's got replaced. Was it the mainboard or something? And if my assumption is right, where exactly all the drivers are loaded since the mainboard is replaced and Windows couldn't detect new hardware with the old drivers? Or is it just because hardware ID's are different???

And if my assumption is right, where exactly all the drivers are loaded since the mainboard is replaced and Windows couldn't detect new hardware with the old drivers? Or is it just because hardware ID's are different???

Yes. Windows sucks at being transferred across machines without being reconfigured.